This application is based on and incorporates herein by reference Japanese Patent Applications No. 11-307931 filed Oct. 29, 1999 and No. 2000-233191 filed Jul. 28, 2000.
The present invention relates to an exhaust gas purification device for an internal combustion engine, in which a plurality of catalysts for exhaust gas purification are disposed in an exhaust gas channel of the internal combustion engine.
In some of recent engines, two catalysts for exhaust gas purification are disposed in series in the exhaust pipe of the engine in order to increase the exhaust gas purification capacity. In such engines, air-fuel ratio sensors (or oxygen sensors) are disposed upstream of the upstream catalyst and downstream of the downstream catalyst, respectively, and the air-fuel ratio of the exhaust gas is feedback controlled to the target air-fuel ratio based on the outputs of these upstream and downstream sensors.
Furthermore, in some of V-type engines, individual exhaust gas passages are provided for each group (each bank) of cylinders and the exhaust gas passages of each group of cylinders are combined downstream in a single collective exhaust gas passage. Respective upstream catalysts are disposed in the exhaust gas passages of each group of cylinders, and the downstream catalyst is disposed in the collective exhaust gas passage. In such engines, air-fuel ratio sensors (or oxygen sensors) are disposed upstream and downstream of the upstream catalyst, and the air-fuel ratio of the exhaust gas is feedback controlled to the target air-fuel ratio based on the outputs of these upstream and downstream sensors.
However, there is a trend toward utilization of catalysts with a high saturated adsorption amount (storage amount) of exhaust gas components with the object of meeting the requirements of exhaust gas regulations that will become increasingly stringent in the future. As a result, the exhaust gas purification systems in which two catalysts are disposed in series in an exhaust pipe have the following drawback. Thus, in a low-load operation mode, or the like, with a low flow rate of exhaust gases, the exhaust gases are sufficiently cleaned by the upstream catalyst alone. Therefore, a long time is required for the changes in the air-fuel ratio of the exhaust gas discharged from the engine to show themselves in the output changes of the sensor located downstream of the downstream catalyst, and the response of the air-fuel ratio control becomes poor.
On the other hand, in the exhaust gas purification system in which upstream catalysts are installed in each group of cylinders, since the sensors are disposed upstream and downstream of the upstream catalysts, a certain response of the air-fuel ratio control can be guaranteed. However, because the air-fuel ratio downstream of the downstream catalyst is not detected, the exhaust gas purification capacity of the whole catalytic system cannot be evaluated and the air-fuel ratio control providing for a full realization of exhaust gas purification capacity of the whole catalytic system cannot be conducted.
It is therefore an object of the present invention to provide an exhaust gas purification device for an internal combustion engine.
It is another object of the present invention to make it possible to conduct an air-fuel ratio control with good response providing for full realization of the exhaust gas purification capacity of the whole catalytic system in a system in which a plurality of catalysts for exhaust gas purification are disposed in an exhaust gas passage.
According to one aspect of the present invention, a plurality of catalysts for exhaust gas purification are disposed in an exhaust gas passage, and sensors are installed for detecting the air-fuel ratio or gas concentration in the exhaust gas upstream and downstream of each of the catalysts. With such a structure, the air-fuel ratio control with good response providing for full realization of exhaust gas purification capacity of the whole catalytic system can be conducted and the exhaust gas purification capacity can be increased by evaluating the current exhaust gas purification capacity (storage amount of each catalyst and the like) based on the outputs of the sensors disposed upstream and downstream of the catalysts. Moreover, the catalyst deterioration determination can be conducted for each of the catalysts.
According to another aspect of the present invention, no less than three catalysts are divided into a plurality of groups of catalysts, each group of catalysts is considered as a single catalyst, and sensors detecting the air-fuel ratio or gas concentration of the exhaust gas are disposed upstream and downstream of each group of catalysts. In such a case, in the system in which no less than three catalysts are disposed in an exhaust gas passage, the air-fuel ratio control with good response providing for full realization of exhaust gas purification capacity of the whole catalytic system can be conducted and the exhaust gas purification capacity can be increased by evaluating the current exhaust gas purification capacity (storage amount of each group of catalysts and the like) for each group of catalysts.